Transformer Calculator

Method

Sizes the overcurrent devices, conductors, grounding and bonding conductors and raceways for a transformer of 1000 V and less, by the 2023 NEC. Each result in the calculator opens to show its steps and the section that requires it.

Transformer

Rated current

I = VA ÷ (√3 × V) three-phase, I = VA ÷ V single-phase, at the winding voltage. Three-phase primaries are taken as delta, 3-wire.

Size from the load

The load in kVA, or amperes at the secondary voltage, plus any spare capacity; the smallest standard rating not less than that.

Fault current

I = I_rated × 100 ÷ %Z at the secondary terminals with an infinite primary source, for the interrupting and short-circuit current ratings of 110.9 and 110.10, with the value at the IEEE C57.12.00 impedance tolerance. A typical impedance is used until the nameplate value is entered. A line-to-neutral fault on a single-phase 3-wire secondary can be about 1.5 times higher.

Overcurrent protection

Primary only

Table 450.3(B): 125 % of rated primary current for 9 A or more, where the next higher standard rating is permitted (Note 1); 167 % below 9 A and 300 % below 2 A, as maximums.

Primary and secondary

Primary device not more than 250 %; secondary device not more than 125 % (next higher standard rating permitted), or 167 % below 9 A.

Panel main, primary only

No secondary device is required by Table 450.3(B), and none is drawn in the one-line. The secondary conductors still end in an overcurrent device, normally the main device at the panel (240.21(C)). It is sized at 125 % of rated secondary current, the next standard rating up, to carry the rated output continuously (215.3), and is shown with the panel.

Ratings

Standard ratings of Table 240.6(A), 2023, which adds 10 A circuit breakers; fuses also 1, 3, 6 and 601 A. Below 15 A, where the rule is a maximum, the fuse ratings made by manufacturers in smaller steps are used.

Conductors

Primary

For continuous loading, 125 % of rated current against the Table 310.16 value at the terminal temperature, before correction and adjustment (215.2(A)(1)(a)), and the rated current against the ampacity after them (215.2(A)(1)(b)). Protected by the primary device (240.4); where the ampacity is not a standard rating, the next higher standard device is permitted up to 800 A (240.4(B)).

Secondary

The same two minimums with the rated secondary current, and an ampacity at least the secondary device rating, without the next-size-up rule (240.21(C)). The length rule adds a minimum: 10 ft, primary device × Vp/Vs not more than 10 × ampacity (240.21(C)(2)(4)); 25 ft, Vp/Vs × one-third of the primary device (240.21(C)(6)(1)); outside, a single device at the disconnect (240.21(C)(4)); 2-wire single-phase or delta-delta, primary device × Vp/Vs (240.21(C)(1), 240.4(F)).

Ampacity

Table 310.16 at the insulation rating, times the ambient correction of Table 310.15(B)(1)(1) (33 °C added on rooftops, except for XHHW-2, 310.15(B)(2)) and the adjustment of Table 310.15(C)(1), and not more than the terminal-temperature column (110.14(C)(1)). By default 60 °C for circuits of 100 A or less and 75 °C above. Small conductors are limited by 240.4(D).

Parallel

Where one conductor up to the chosen largest size is not enough, sets of 1/0 AWG and larger in separate raceways (310.10(G)). The smallest number of sets, then the smallest size.

Neutral

The same size as the phase conductors by default; never smaller than Table 250.102(C)(1), and 1/0 AWG in parallel (250.30(A)(3)). It counts as a current-carrying conductor for nonlinear loads (310.15(E)(3)).

Grounding and bonding

Primary EGC

Table 250.122 for the primary device; not larger than the circuit conductors (250.122(A)); one in each parallel raceway (250.122(F)(1)). Increased by the ratio of circular mils where the phase conductors are larger than the minimum, as for voltage drop (250.122(B)); the ratio uses the total area of each phase, and sizes within 0.1 % (the rounding of Table 8) are accepted.

Secondary

Separately derived system. Supply-side bonding jumper with the secondary conductors (250.30(A)(2)), system bonding jumper (250.30(A)(1)) and grounded conductor (250.30(A)(3)) by Table 250.102(C)(1). Above 1100 kcmil copper or 1750 kcmil aluminum, 12.5 % of the area (Note 1). The jumper in each parallel raceway uses the conductors in that raceway (250.102(C)(2)). Grounding electrode conductor by Table 250.66 (250.30(A)(5)). Delta 3-wire, 277 V and 480 V 2-wire secondaries are not required to be grounded and are shown as ungrounded systems (250.20(B), 250.30(B)); a 240 V 2-wire secondary is grounded at its midpoint, 120 V to ground (250.20(B)(1)).

Raceways and voltage drop

Fill

Conductor areas from Chapter 9 Table 5 (Table 8 for bare), grounding and bonding conductors included (Note 3); 53, 31 or 40 % of the Table 4 area for one, two or more conductors (Table 1). The smallest trade size of the chosen raceway. A jam ratio of 2.8 to 3.2 is flagged.

Voltage drop

Ze = R cos θ + X sin θ from Chapter 9 Table 9 at 75 °C (PVC, aluminum or steel conduit); VD = √3 I Ze L / 1000 three-phase, 2 I Ze L / 1000 single-phase, at the rated current. The NEC suggests 3 % for a feeder and 5 % in total (215.2(A) Informational Note). Sizes can be increased to meet a limit.

Checks

Each case is solved live by this calculator and compared with a published worked example.

Limitations

• Transformers of 1000 V and less (Table 450.3(B)). Primaries over 1000 V (Table 450.3(A)) are not covered.
• Feeder taps ahead of the transformer (240.21(B)), the industrial secondary rule (240.21(C)(3)) and the secondary of a feeder-tapped transformer (240.21(C)(5)) are not covered.
• The calculation treats the rated output as the load. The neutral load, harmonics and the downstream feeder are the designer's to confirm.
• Terminal temperature, equipment ratings and the fault current with the source impedance (Fault Current Calculator) should be confirmed for the actual equipment.